Bend-insensitive long period fiber grating-based high temperature sensor

• An ultra short long period fiber grating with only two periods is fabricated.
• Bend and high temperature characteristics of the proposed grating are investigated.
• The bend sensitivity is three orders lower than those of the conventional gratings.
• The ultra short grating could survive under high temperature of 1000 °C.

A novel bend-insensitive long period fiber grating (LPFG) is presented and applied in high temperature measurement. This LPFG is formed by periodically arranging three micro-core-offsets fabricated by employing the cleaving–splicing method in the standard single mode fiber. As bend is applied onto the LPFG along the most bend-insensitive orientation, the resonant wavelength sensitivity of the transmission spectrum is 0.0097 nm/m−1 for a curvature range of 0–3.5 m−1, which is about three orders lower than those of the conventional gratings. For other orientations of the LPFG, the bend sensitivities are also one or two orders lower than those of conventional ones. This better bend characteristic could be used to improve the sensing performances of the LPFG, such as improving the measurement accuracy and resolving cross sensitivity issue between bend and other parameters. Moreover, since the mode coupling of the LPFG results from the fiber geometry deformation which indicates a better thermal stability, the LPFG could be used for high temperature measurement occasions.